WEBVTT 00:00.000 --> 00:01.000 As the first blood cells 00:01.033 --> 00:03.333 appear in the yolk sac, 00:03.367 --> 00:04.767 blood vessels form 00:04.800 --> 00:07.000 throughout the embryo, 00:07.033 --> 00:11.800 and the tubular heart emerges. 00:11.833 --> 00:13.467 Almost immediately, 00:13.500 --> 00:14.900 the rapidly growing heart 00:14.933 --> 00:16.867 folds in upon itself 00:16.900 --> 00:18.033 as separate chambers 00:18.067 --> 00:21.900 begin to develop. 00:21.933 --> 00:23.200 The heart begins beating 00:23.233 --> 00:24.633 3 weeks and 1 day 00:24.667 --> 00:27.767 following fertilization. 00:27.800 --> 00:30.600 The circulatory system is the first body system, 00:30.633 --> 00:32.433 or group of related organs, 00:32.467 --> 00:34.700 to achieve a functional state.